Corresponding dryers equipped with an extruder unit are known from the state of the art and are generally used for drying pasty biomasses such as (sewage) sludge. To achieve this, the drying surface and extruder unit are movably mounted relative to one another, whereby the relative movement is coordinated in such a way that the biomass is deposited on the drying surface in the shape of a serpentine line and as a continuous strand. An air supply, in turn, is assigned to drying surface used for moving warm and/or dried drying air through the drying surface provided with the corresponding air passages. In this case, the drying air makes contact with the biomass and dries it until it becomes the desired dry matter.
Usually, the extruder unit has a perforated plate with many apertures through which the biomass is pressed through by applying the corresponding excess pressure in order to give it the desired strand shape, whose considerable surface facilitates the drying of the biomass.
Since slurry, in particular, contains—apart from unproblematic fine components—also coarse particles (fibers, hairs, etc.) that could clog or entwine the apertures of the perforated plate, it has proven useful to equip the extruder unit with a blade unit which, in turn, has one or several scraping blades movable relative to the perforated plate with the help of a drive, for shearing off the coarse particles that have accumulated in the apertures.
It has been shown, however, that the known blade units are no longer capable of securing the smooth operation of the extruder unit when the biomass being treated has high solid content (over 30% of dry matter) because coarse particles such as highly dense slurry lumps, which owing to their size do not get caught in the apertures of the perforated plate, are moved by the blade unit along the surface of the perforated plate in the manner of a snowplow. As a result of that, the full opening for the biomass to pass through is no longer available (as the coarse particles cover a part of it) and this ultimately reduces extrusion effectiveness.